I can see how you didn't get a hold of them calling them, but you emailed them and they haven't responded in a few days?
What's up with that?
If the vendor has half a brain, they'll figure it out themselves (tracking will show delivery and who signed for it). If they shipped it without tracking (USPS) they're SCREWED!
Don't let your conscience bother you though. LEGALLY you're not obligated to do squat. Sure, morally you might feel you are and the company might perk you for being cool.
I actually did this once. Had a guy send me a motherboard and a hard drive for exchange. The motherboard was good (don't know about the HDD since it was so badly packaged that it could've died in transit), so I sent the motherboard back to the guy... or so I thought. I shipped it to a guy with the same name, different guy. :Q The guy who got the board called and I issued a call tag. The guy who was SUPPOSED TO get the board got a call telling him what had happened and that a new board would be shipped UPS 2 DAY. The second guy insists on a new board shipped UPS Next Day with Saturday Delivery. I remind him that he returned a perfectly good motherboard assuming it was bad. He said he didn't care. I said "then you won't care if I don't send you anything and just charge you a 15% restocking fee for sending back good product."
Needless to say, he quit pushing his luck.
Money For Nothing (Dire Straits)